Just about any fish besides oto's will eat shrimp, as long as they can fit them into their mouths. The snails is a different beast all together, certain loaches eat snails, but I'm not sure which ones will eat assassins, probably have to be one of the bigger ones. Assassin snails only reproduce when there is a fresh supply of live food, so if you don't introduce any other snails, you're population will eventually die off.